Combining pure liquid and vapor nitrogen streams from air separation for crude hydrogen gas washing



Feb. 11, 1969 R. BECKER 3, 6, 3

COMBINING PURE LIQUID AND VAPOR NITROGEN STREAMS FROM AIR SEPARATIQN FORCRUDE HYDROGEN GAS WASHING Filed 061;. 1.7, 1967 *[U 1755:. Wig-N 80 mBrigas 58%78 Yic lnvenfor RUDOLF BECKER A #o/nep United States Patent 7Claims priority, application Gsermany, June 19, 1963,

U.S. Cl. 62-13 Int. Cl. F25j 3/04 21 Claims ABSTRACT OF THE DISCLOSUREAir is separated in an air separation plant employing a doublerectification column. An impure nitrogen stream from the high pressurecolumn is recompressed and then cooled by vaporizing oxygen from the lowpressure column as well as exiting ammonia synthesis gas and scrubberliquid streams from a crude hydrogen scrubber before returning to thehigh pressure column. Separate streams of pure nitrogen liquid and vaporare removed from the high pressure column which streams are combined andcompletely liquefied with part of the liquid nitrogen being used to washa crude hydrogen gas while another part of the pure liquefied nitrogenis added to the washed hydrogen to form a gas stream for ammoniasynthesis.

This is a continuation-in-part of copending US. patent application Ser.No. 373,418, filed June 8, 1964, now abandoned.

This invention relates to a process for air fractionation and isparticularly concerned with the use of a highly compressed gas in coldcirculation for pressure vaporization of liquid oxygen, and with the useof a group of periodically transposable regenerators for heat exchangebetween the air to he fractionated and the oxygen.

Besides the production of oxygen, the invention is also concerned withthe separation of pure nitrogen, especially for the production ofsynthetic ammonia.

The invention is carried out in a very simple and economical manner bytaking from the rectification column an impure fraction of circulatingnitrogen, warming it to the temperature of the surroundings by passagethrough a regenerator which has been cleansed by scavenging withnitrogen during at least one preceding stage, liquefying the impurefraction by heat exchange after high compression and, preferably, aftercleansing in an adsorber, and then, after passage through an expansiondevice, returning the impure nitrogen to the rectification column at apoint near the place from which it was taken, whereby pure nitrogen isobtained from the head section of the column, preferably for independentuse.

The impure nitrogen is preferably removed from the middle section of thepressure column and, after being at least partly liquefied, is returnedto the same place.

For the production of the required additional cold, a portion of theimpure nitrogen fraction that was taken from the middle section of thepressure column, after a preliminary warming, is expanded while doingwork and is then used as a scavenging gas for the aforementionedscavenging step for the regenerators. The gas to be expanded can betaken, after sutfi-cient prewarming, from a suitable place on theregenerator through which circulating nitrogen is passed, and/ or if itbypasses this regenerator, is prewarmed by passage through a heatexchanger with circulating nitrogen before being returned to thepressure column.

Patented Feb. 11, 1969 "ice Another feature of this invention is thatthe pure nitrogen is taken from the head of the pressure column.

If the pure nitrogen is to be used for the synthesis of ammonia, it isadvantageous to take the pure nitrogen from the head of the column in apartly liquid and partly gaseous state, in preferred proportions of 1part liquid to 6-20 parts gas, the most preferred proportion being 1part liquid to 10 par-ts gas; and after complete liquefaction withfurther cooling under the same pressure if necessary, it is subjected toheat exchange with possibly expanded products of a synthetic ammoniaplant, is then compressed to the pressure of the washing column of thatplant, and is used as a washing fluid for hydrogen gas and as acomponent of the ammonia synthesis gas. In this manner the necessity ofcompressing the gaseous nitrogen to the normally higher pressure of thewashing column of the synthetic ammonia plant is avoided, and the pumpfor compressing the liquefied nitrogen uses much less energy.

The process of this invention will now be described more in detail withreference to the schematic drawing illustrating the process andapparatus of the invention.

The air to be fractionated enters by conduit 1, is passed throughcompressor 2 to compress it to about 6 atmospheres, and arrives byconduit 3 at regenerator 4, which is one of the series of transposableregenerators 4 to 7, through valve 411, which is one of the series ofvalves 4a to 7a that control communication between conduit 3 and theregenerators. The deeply cooled air passes through a transposable valveor part of double check valve 4a, which is one of the series of suchvalves 4d to 7d for delivery through conduit 8 to the lower part ofpressure column 9 which, in conjunction with the low pressure column 10,forms a double rectifier tower arrangement.

The liquid in the sump of the pressure column, which is under a pressureof about 5.6 atm., is delivered from the bottom of the column by conduit11 to adsorber 12, thence to heat exchanger 13 from Where it passesthrough conduit 14 and expansion valve 15 to the middle portion of thelow pressure column 10 in which the pressure is about 1.4 atm.

From the middle portion of the pressure column 9 an impure nitrogenfraction is removed by conduit 16 and is passed through the transposablevalve 72, which is one of the series of valves 4e to 7e, for delivery toregenerator 7 of the series 4 to 7. The greater portion of this fractionpasses through transposable valve 7c, which is one of the series ofvalves 40 to 7c, and then through conduit 17 to the compressor 18 forcompression to about atm.

The compressed gas leaves the compressor via conduit 19, and preferablypasses through an adsorber 20 having a bypass valve 21, to a group ofheat exchangers 23, 24 and 25 and from there passes through thecollecting conduit 26 to heat exchanger 27.

From heat exchanger 27, the circulating nitrogen passes on the one sidethrough conduit 28 and through heat exchangers 29 and 30 and throughconduit 31 to the collecting conduit 32, while on the other side itpasses through conduit 33, heat exchanger 34 and conduit 35 to the samecollecting conduit 32.

In the conduit 32 there is a throttle valve 36 through which thenitrogen, which is now at least partl liquefied, passes for return tothe same portion of the pressure column 9.

Above the place of entry of the conduit 32 in column 9 there is mounteda liquid-collecting basin 37 from which the conduit 38 leads to theoutside, delivering liquid nitrogen through the heat exchanger 39,conduit 40 and expansion valve 41 to the head of the low pressure column10.

From the head of the low pressure column 10 nitrogen is withdrawnthrough conduit 42 and through heat exchanger 39, conduit 43, heatexchanger 13 and conduit 44 3 and through the two valves 5d and 6d ofthe series M to 7d, and passes as scavenging gas through twotransposable regenerators 5 and 6 of the series 4 to 7 in which it iswarmed to the temperature of the surroundings. The scavenging gas thenpasses through the two valves 51) and 6b of the series 4/) to 7b andmakes its exit through conduit as impure nitrogen gas.

The periodic reversal of the regenerators 4 to 7 is performed in such amanner that each regenerator in a first phase of operation is traversedin a downward direction by air to be cooled therein to a low temperaturein preparation for its fractionation, in a second and third phase in anupward direction by scavenging gas which is to be warmed therein whileeffecting sublimation of the CO and of ice formations, and in a fourthphase in an upward direction by circulating nitrogen that is to bewarmed.

A portion of the circulating nitrogen fraction is advantageouslydiverted from the lower half of the corresponding regenerators throughvalve 7 f of the series of valves 4] to 7f and is delivered throughconduit 46 having valve 47 therein and through conduit 48 to theexpansion turbine 49. The expanded gas then passes through conduit 50 tothe collection conduit 44 for scavenging gas. Additionally, oralternatively, a portion of the impure nitrogen can be diverted from thecirculation conduit 16 by Way of conduit 51 for passage through heatexchanger 34 and conduit 52 having therein valve 53 and then throughconduit 48 to the expansion turbine 49. The regulation is determined bythe temperature equilibrium which must be maintained during the process.

From the sump of the low pressure column 10 liquid oxygen is removed byconduit 54, is compressed to about 41 atm. by pump 55, and is deliveredb conduit 56 to a series of heat exchangers 29, 27 and 23 in which theliquid oxygen is brought to about the temperature of the surroundingsfor delivery under about 40 atm. by conduit 57 having therein valve 58to its destination.

From the head of the pressure column 9 pure gaseous nitrogen is removedby conduit 59 while liquid nitrogen is removed by conduit 60 under apressure of about 5.5 atm., both of these conduits delivering theirnitrogen to the heat exchanger 61 in which most of the remaining gaseousnitrogen is liquefied. The largely liquefied nitrogen is then deliveredby conduit 62 to heat exchanger 63 where it is further cooled and isdelivered by conduit 64 to pump 65. Pump 65 then delivers the liquidnitrogen through conduit 66 having therein valve 67 to the head of theWashing column 68 under a pressure of about 25 atm.

A portion of the liquid nitrogen is diverted from conduit 66 by conduit69 with valve 70 directly to conduit 71 which draws gas from the head ofthe washing column 68 to produce a stoichiometric mixture for ammoniasynthesis. The conduit 71 then delivers the hydrogen-nitrogen mixturethrough heat exchangers 61 and 72, conduits 73 and 74, heat exchangers75 and 25, and then through conduits 76 and 77 to the collecting conduit78 having therein valve 79 for delivery of the hydrogen-nitrogen mixtureto an ammonia synthesis plant under a pressure of about 24 atm.

A hydrogen rich gas such as converter gas is delivered by conduit 80 tocompressor 81, and thence by conduit 82 and heat exchanger 83, which canbe water-cooled, and by conduit 84 to heat exchanger 75, then throughconduit 85 to heat exchanger 72, and from there through conduit 86 toheat exchanger 87. From heat exchanger 87 the gas is delivered byconduit 88 to the cooling coil 89 in heat exchanger 63 and from there byconduit 90 to the lower portion of the washing column 68.

The liquid nitrogen with impurities consisting mainly of CO is takenfrom the sump of the washing column 68 by conduit 91 with expansionvalve 92 therein for passage through heat exchangers 63 and 87, conduit93, heat exchanger 30, conduit 94, heat exchanger 24, and then toconduit 95 for delivery to the Outside.

Referring to the drawing, the following table is illustrative ofquantities found in the various conduits:

C onduit Fluid description Quantity 80,000 N11L /l1. 30,000 Nm. /l1.12,500 Nmfi/h. 35,000 Nmfi/h. 42,000 Nmfi/h. 10,000 Nmfi/h. 1,000 Nmn'/h.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions. Consequently, such changes and modifications are properly,equitably, and intended to be, within the full range of equivalence ofthe following claims.

What is claimed is:

1. In a process of fractionating air and producing ammonia synthesis gasby washing raw hydrogen containing carbon monoxide with pure liquidnitrogen wherein said air is fractionated in a double column having ahigh pressure section and a low pressure section to produce said pureliquid nitrogen, the improvement which comprises withdrawing purenitrogen fluid from the top zone of the high pressure section of thedouble column in two separate streams, one being substantiallycompletely in the liquid phase, and the other being substantiallycompletely in the vapor phase in proportions of 6-20 parts vapor to 1part liquid, combining and completely liquefying said combined fluidstreams by heat exchange with one or more streams from the ammoniasynthesis gas production, said streams being (1) pure ammonia synthesisgas, (2) crude hydrogen, and (3) a mixture of nitrogen and carbonmonoxide.

2. A process as defined by claim 1 wherein streams (1), (2), and (3) areheat exchanged with the pure nitrogen fluid.

3. In a process of fractionating air and producing ammonia synthesis gasby washing raw hydrogen containing carbon monoxide with pure liquidnitrogen wherein said air is fractionated in a double column having ahigh pressure section and a low pressure section to produce said pureliquid nitrogen, the improvement which comprises withdrawing purenitrogen fluid from the top zone of the high pressure section of thedouble column in two separate streams, one being substantiallycompletely in the liquid phase, and the other being substantiallycompletely in the vapor phase in a proportion of 10 parts vapor to 1part liquid, combining and completely liquefying said combined fluidstreams by heat exchange with one or more streams from the ammoniasynthesis gas production, said streams being (1) pure ammonia synthesisgas, (2) crude hydrogen, and (3) a mixture of nitrogen and carbonmonoxide.

4. In a process of fractionating air and producing ammonia synthesis gasby washing raw hydrogen with liquid nitrogen wherein said air isfractionated in a double column having a high pressure section and a lowpressure section, the improvement which comprises withdrawing impurenitrogen from the high pressure section, heating said impure nitrogen byindirectly transferring heat between said impure nitrogen and raw air,compressing resultant heated impure nitrogen, cooling resultantcompressed impure nitrogen in indirect heat exchange with one or morestreams from the ammonia synthesis gas production, said streamscomprising (a) a mixture of nitrogen and carbon monoxide, and (b) pureammonia synthesis gas; and recycling resultant cooled impure nitrogen tothe double column.

5. A process as defined by claim 4 wherein said impure nitrogen iswithdrawn from a middle section of said high pressure section.

6. A process as defined by claim 4 wherein said indirect heat exchangeof compressed impure nitrogen is conducted with streams (a) and (b).

7. A process as defined by claim 4 wherein said indirect heat exchangeof compressed impure nitrogen is conducted with streams (a) and (b).

8. A process as defined by claim 4, further comprising withdrawing purenitrogen fluid from the high pressure section of the double column,partly in the vapor and partly in the liquid state, and completelyliquefying said fluid by heat exchange with one or more streams from theammonia synthesis gas production, said streams being (1) pure ammoniasynthesis gas, (2) crude hydrogen, and (3) a mixture of nitrogen andcarbon monoxide.

9. A process as defined by claim 8 wherein streams (1), (2), and (3) areheat exchanged with the pure nitrogen fluid.

10. A method of fractionating air comprising: compressing air andpassing the compressed air in one direction through a first regeneratormeans to the high pressure part of a double rectifying column near thebottom thereof, withdrawing liquid from the bottom of said high pressurecolumn and delivering it to an intermediate point in the low pressurepart of said double rectifying column, withdrawing nitrogen from the topof said low pressure column and passing said nitrogen through at least asecond regenerator means to be scavenged in the direction opposite tosaid one direction, withdrawing impure nitrogen from a middle section ofsaid high pressure column at a point wherein said impure nitrogen is notof sufficient purity for ammonia synthesis, and passing the impurenitrogen through a further regenerator means in the direction oppositeto said one direction, compressing at least a part of the impurenitrogen after passing the same through said further regenerator means,drawing liquid oxygen from the bottom of said low pressure columncompressing said liquid oxygen and exchanging heat between theevaporating compressed oxygen and the compressed impure nitrogen, thelatter being thereby at least partly liquefied, and returning the atleast partly liquefied impure nitrogen to said high pressure column nearthe point of withdrawal of the impure nitrogen from the column, andwithdrawing pure gaseous nitrogen from the top of a condenser-reboilerin the high pressure column, the purity of said pure gaseous nitrogenbeing suflicient for ammonia synthesis gas.

11. A process as defined by claim 10, further comprising withdrawingliquid nitrogen of sufiicient purity for ammonia synthesis from the topzone of the high pressure column, said withdrawing being at a pointabove the point for said withdrawing of impure nitrogen.

12. A method as defined by claim 10 further comprising the steps ofwithdrawing pure condensed liquid nitrogen from the top zone of the highpressure column, and combining said liquid with said pure gaseousnitrogen for flow to ammonia synthesis.

13. A process as defined by claim 12, wherein both said pure gaseousnitrogen and said pure condensed liquid are of suflicient purity forammonia synthesis gas, the point of withdrawing said pure condensedliquid being above the point of withdrawing said impure nitrogen, saidpure gaseous fraction and said pure liquid fraction being passed to theammonia synthesis plant without any further rectification.

14. A method of fractionating air comprising: compressing air andpassing the compressed air in one direction through a first regeneratormeans to the high pressure part of a double rectifying column near thebottom thereof, withdrawing liquid from the bottom of said high pressurecolumn and delivering it to an intermediate point in the low pressurepart of said double rectifying column, withdrawing nitrogen from the topof said low pressure column and passing said nitrogen through at least asecond regenerator means to be scavenged in the direction opposite tosaid one direction, withdrawing impure nitrogen from a middle section ofsaid high pressure column and passing the impure nitrogen through afurther regenerator means in the direction opposite to said onedirection, compressing at least a part of the impure nitrogen afterpassing the same through said further regenerator means, drawing liquidfrom the bottom of said low pressure column, compressing said liquidoxygen and exchanging heat between evaporating compressed oxygen and thecompressed impure nitrogen thereby at least partly liquefied, andreturning the impure nitrogen to said high pressure column near thepoint of withdrawal of the impure nitrogen from the column, withdrawingpure nitrogen from the head of said high pressure column partly as aliquid and partly as a gas, completely liquefying the withdrawn purenitrogen by heat exchange with one or more streams from an ammoniasynthesis gas plant, said streams being (1) pure pure ammonia synthesisgas, (2) crude hydrogen, and (3) a mixture of nitrogen and carbonmonoxide. pumping the resultant liquefied nitrogen to an increasedpressure, washing said hydrogen gas with a part of the nitrogen, andadding another part of the nitrogen to the washed gaseous hydrogen toproduce ammonia synthesis gas.

15. A process as defined by claim 14, further comprising exchanging heatbetween said compressed impure nitrogen and one or more streams from aplant for producing ammonia synthesis gas by scrubbing crude hydrogenwith liquid nitrogen, said streams comprising (a) a mixture of nitrogenand carbon monoxide, and (b) pure ammonia synthesis gas.

16. A process as defined by claim 15 wherein both streams (a) and (b)are heat exchanged with said compressed impure nitrogen.

17. A process as defined by claim 16 wherein streams (l), (2), and (3)are heat exchanged with pure nitrogen.

18. A process as defined by claim 14 wherein streams (1), (2), and (3)are heat exchanged with pure nitrogen.

19. A method of fractionating air comprising: compressing air andpassing the compressed air in one direction through a first regeneratormeans to the high pressure part of a double rectifying column near thebottom thereof, withdrawing liquid from the bottom of said high pressurecolumn and delivering it to an intermediate point in the low pressurepart of said double rectifying column, withdrawing nitrogen from the topof said low pressure column and passing said nitrogen through at least asecond regenerator means to be scavenged in the direction opposite tosaid one direction, withdrawing impure nitrogen from a middle section ofsaid high pressure column and passing the impure nitrogen through afurther regenerator means in the direction opposite to said onedirection, compressing at least a part of the impure nitrogen afterpassing the same through said further regenerator means, exchanging heatbetween said compressed impure nitrogen and one or more streams from aplant for producing ammonia synthesis gas by scrubbing crude hydrogenwith liquid nitrogen, said streams comprising (a) a mixture of nitrogenand carbon monoxide, and (b) pure ammonia synthesis gas, drawing liquidoxygen from the bottom of said low pressure column compressing saidliquid oxygen and exchanging heat between the evaporating compressedoxygen and said compressed impure nitro gen thereby at least partlyliquefied, and returning the latter impure nitrogen to said highpressure column near the point of withdrawal of the impure nitrogen fromthe column, withdrawing pure nitrogen from the top zone of the highpressure column for separate use.

20. A process as defined by claim 19 wherein both streams (a) and (b)are heat exchanged with said compressed impure nitrogen.

21. The combination, in an apparatus for fractionating air; firstregenerator means, means to compress air and pass it through said firstregenerator means to chill the air, a double rectification column with ahigh pressure and a low pressure portion connected near the bottom ofthe high pressure portion to said first regenerator means to receive thechilled air therefrom, at least a second regenerator means, means forwithdrawing nitrogen from the head of said low pressure column and forpassing said nitrogen through said at least second regenerator means toscavenge said regenerator means, means for withdrawing impure nitrogenfrom a middle section of said high pressure rectifying column and forpassing said impure nitrogen through a further regenerator means to bewarmed, means for compressing the warmed impure nitrogen employed forcooling the further regenerator means, means for at least partlyliquefying the compressed nitrogen, and means including a throttle valvefor returning the at least partly liquefied nitrogen to said column nearthe point thereof from which it was withdrawn from the column, conduitmeans leading from the head of the high pressure portion of said columnfor drawing pure nitrogen therefrom, said conduit means comprising twoseparate pipes, one for withdrawing liquid-phase pure nitrogen, and theother for withdrawing vapor-phase pure nitrogen, heat exchanger means towhich said conduit means lead operable for effecting completeliquefication of said nitrogen, pump means connected to the heatexchanger means to References Cited UNITED STATES PATENTS 2,895,3047/1959 Wucherer et al 6213 2,936,593 5/1960 Grunberg 62-20 2,962,86712/1960 Seidel 62-13 XR 3,216,206 11/1965 Kessler 6213 2,526,996 10/1950Crawford 62--13 XR NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

US. Cl. X.R.

